White anodising (Topic: 14124)

Production of for instance red, brown, black, or yellow colours are obtained by electrolytic colouring or dyeing. A combination of these two production types is also used to produce other particular colour shades, but real white colouring of the anodic oxide coating has nobody been able to produce. The market demand for white anodised profiles for architectural work like window frames is enormous, but white powder coated profiles have to be used instead.

Many different approaches have been tried, including the use of special alloys, chemical etching methods, two-stage anodising, electrolytic colouring, and the use of the current recovery process occurring when the anodising voltage is suddenly lowered.

The possibilities of white anodising have attracted much interest in Japan. One process gave a 5 µm thick film with an opaque enamel like coating. But, in common with all the other white oxide coatings, the coating has not a real white appearance, and the outdoor stability is often very poor.

Electrophoretic painting has been tried, but the white pigments are to big to penetrate down into the pores obtained in a sulphuric acid electrolyte ,and will only stick to the pore-openings. The result is just a kind of electrophoretic sealing. The positive effect is that a good adhesion is obtained.